Dispersion strengthened or reinforced metal matrix composites have been developed for aerospace, automobile and other applications where light-weight materials with improved physical (mechanical) properties, such as tensile strength, elongation, wear resistance, etc., are required. Metal matrix composites generally comprise solid reinforcing particles, such as ceramic or graphite reinforcing particles, dispersed uniformly throughout a metal matrix. The reinforcing particles may assume various forms including fibers, whiskers, rods, spheres and the like, and may be present in the metal matrix in amounts up to 50 volume percent depending upon the physical properties desired for the metal matrix composite.
In forming dispersion strengthened or reinforced metal matrix composites, ingots of the matrix metal are typically melted in an induction furnace and the solid reinforcing particles are added in the desired amount to the molten metal to form a two-phase melt. As a result of induction stirring of the melt, the reinforcing particles are initially uniformly dispersed throughout the melt.
However, when the melt is subsequently cast in a permanent mold or a sand mold and solidified, a tendency for the solid reinforcing particles to agglomerate and form clusters in the casting has been observed. The presence of clusters of reinforcing particles in the cast metal matrix composite is highly undesirable as such clusters adversely affect the physical properties of the composite and/or their uniformity throughout the composite.
The invention resulted from the discovery that the objectionable clumping or agglomeration of the solid reinforcing particles in the cast metal matrix composite occurs in the relatively short time between transfer of the melt from the induction furnace and casting of the melt in the mold and also during solidification of the melt in the mold.
It is an object of the invention to provide a method of casting a metal matrix composite from an initially homogeneous, two-phase melt of solid reinforcing particles in a molten metal in such a manner as to minimize subsequent clumping or agglomeration of the reinforcing particles in the melt during various stages of the casting process and thus to minimize the presence of objectionable clusters of the reinforcing particles in the cast metal matrix composite.
It is another object of the invention to provide a method of casting a metal matrix composite by countergravity filling a mold cavity in a mold from an underlying, initially homogenous, two-phase melt of solid reinforcing particles in a molten metal while stirring the melt to minimize agglomeration or clumping of the reinforcing particles therein during countergravity mold filling.
It is a further object of the invention to stir the melt in the mold cavity after countergravity filling thereof and preferably even during solidification of the melt in the mold cavity to further minimize the presence of clumps or clusters of reinforcing particles in the solidifying melt.